This episode marked the first time that Ianto Jones played a major part in the series. It also marked the first appearance of a villain from Doctor Who. Additionally, Gwen Cooper discovered that Owen Harper had developed a sexual attraction to her in a not-so-subtle way, and this powerful lust for her would soon become a temptation that put a wedge in her relationship with Rhys Williams.

During a mundane day at Torchwood Three, the team decide to go get a drink, but Ianto politely stays behind. Once alone, he brings a Japanesedoctor named Tanizaki into the Torchwood Hub and takes him to a holding cell deep below the surface. There, a partially Cyber-converted woman is strapped to a conversion unit surrounded by instruments keeping her alive. Ianto introduces her to Tanizaki as Lisa Hallett, his lover. They both worked at Torchwood One in London before its fall in the Battle of Canary Wharf. Lisa was only partially converted during the battle, and Ianto managed to get his half-Cybernetic girlfriend to safety afterwards. He rigged the equipment in secret to keep her alive, hiding her in the Hub's basement in hopes of curing her. He hopes Dr Tanizaki, a cybernetics expert, can undo the conversion and make her human again.

They take her to the autopsy lab and let her breathe on her own, but soon the rest of the team return to deal with a rogue UFO. Ianto leaves Dr Tanizaki to take Lisa back to the cell while he prepares for the team's arrival. After he leaves, Lisa's Cyberman influence takes over. She attempts to "upgrade" Dr Tanizaki to repay him for his favour, killing him.

As the rest of Torchwood deal with the Arcan leisure crawler that's entered their atmosphere, there is a brief power flicker; Ianto covers up by claiming that the generators have been acting up all day and offers to look into it. Returning to Lisa's cell, he finds Dr Tanizaki's body bloodied and mutilated after a failed cyber-conversion, with Lisa standing over him. Ianto tells Lisa to stay in the cell as he drags off the corpse to hide it from the team. Lisa re-enters the cybernetic unit and begins to drain more power from Torchwood, soon noticed by the others.

Arming themselves, Gwen and Owen head for the holding cell where the power drain is occurring, believing themselves under attack. Once there, they find and Owen recognises the cyber-conversion unit, but are attacked by Lisa. Owen is knocked out. Gwen points her gun, and tries to reason with Lisa, who knocks the gun out of her hand and pushes her over. Gwen lands on her back on the conversion unit. Before she can sit up, Lisa straps her down. Jack tries to shoot Lisa before she can start the unit up, but Ianto tackles him, allowing Lisa to escape after starting up the machine, leaving Gwen powerless as the machine's saws and knives get closer to her head.

Gwen is terrified and yells for help. Jack tries to shut off the conversion unit but Lisa had altered the machine. He calls Tosh and tells her to shut power off to the base. Tosh does this, which unfortunately also means entering total lock-down. The team regroup in the Hub as the power is drained further by Lisa to recreate a new army of Cybermen. Jack demands that Ianto tell them everything. Ianto pleads that since he cleans up after Torchwood but is never asked about his life, they should help him restore Lisa, but Jack maintains that there is no cure. Ianto asks that he be permitted to reason with Lisa before the team attempts to attack her again.

Myfanwy attacks Lisa.

Lisa appears in the Hub and Ianto tries to remind Lisa she is still human, but she is disgusted by her partially-human appearance. She offers to implant her brain into Ianto, believing that sharing the same life is true love, but Ianto refuses. Lisa therefore declares the two incompatible and throws him aside, knocking him unconscious and sending the rest of the team into action. Jack orders Toshiko to go to the surface with emergency power cells so they can open the weapons lockers while Gwen and Owen are to search for any weapons. He will try to delay Lisa's advance. Lisa electrocutes Jack twice, to Gwen's and Owen's shock.

Gwen and Owen are momentarily trapped by Lisa in the autopsy lab, and hide in a locker. As Lisa searches for them, Gwen and Owen kiss. Gwen's phone rings, alerting Lisa to their hiding place and Gwen and Owen to try to escape. As Lisa corners Gwen, Owen grabs a scalpel and stabs Lisa in the chest seemingly killing her. When Lisa starts to revive, they escape to the main Hub where they find Jack, very much alive. Jack sprays Lisa with a special "barbecue sauce" that helps their pet pterodactyl identify its prey. As it attacks Lisa, the rest of the team, along with the recovered and protesting Ianto, escape through the invisible lift. Upon exiting the lift, Ianto punches Jack. Tosh runs up to them and tells Jack that she rigged the doors to open after a few minutes and Ianto runs to go back inside. At the entrance to Torchwood, Jack and the rest of the Torchwood group catch up to Ianto who pulls a gun on them and threatens to shoot them if they try to stop him. Jack knocks the gun away, pulling his own and threatens to shoot Ianto, warning that if he fails to kill Lisa within ten minutes, he will come down and kill them both. Ianto returns to Torchwood by himself.

Ianto's anguish over losing Lisa.

Unbeknownst to the team, a pizza delivery girl, using Torchwood's other entrance to bring some pizzas ordered earlier by Ianto, finds the pterodactyl on the floor. Then Lisa is suddenly behind her. As Ianto makes his way to the holding cell, he finds Lisa's body has died. The delivery girl, appears calling Ianto. She has a large cut across her forehead, claims she is Lisa and she took the girl's body so they could be together. She begs Ianto to hold her. Crying, Ianto hugs her, then pushes her away and holds her at gunpoint. Lisa tries to explain she did this for Ianto. He cries and fails to shoot. She promises they can be upgraded together. The rest of the Torchwood team arrive just in time to hear that, and open fire on the girl, killing her and leaving Ianto to mourn over Lisa's body.

The next day, Ianto enters the Hub. Jack and Gwen watch him cleaning up and they discuss how Ianto couldn't bear to live without Lisa. Gwen asks Jack if he would have shot her if she had stood by Ianto. Jack simply states that she didn't. When she asks, he refuses to tell Gwen that he'd ever loved anyone to the extent that Ianto loved Lisa. They continue to watch in silence.

Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.